Punctiform cathode, in particular suitable for detachable electric discharge tubes

ABSTRACT

A punctiform cathode for detachable tubes consisting of a tantalum plate which is secured to the tops of one or more Vshaped filaments.

United States Patent;

Field of Search. ..3l3/2l7, 311, 336, 341, 343

van Esdonk 1 Sept. 5, 1972 [54] PUNCTIFORM CATHODE, IN

PARTICULAR SUITABLE FOR [56] 7 References Cited $$HABLE ELECTRIC DISCHARGE UNITED STATES PATENTS I 72 l t hams 2,126,894 8/1938 Knoll et a]. ..313/ 336 1 men or 11 Esdonk Emmasmgel 2,151,803 3/1939 Rust et al ..3l3/336 x 3,209,197 9/1965 Ahsmann et al ..3l3/3ll x [731 Asslgnw Philips Common New 3,374,386 3/1968 Charbonnier er al. .313/336 x Ymk, 3,461,338 8/1969 Vogel ..313/33 x [22] Filed: Feb. 22, 1971 d S h b Primary Examiner-Davi c on erg [21] Appl' 7501 Assistant Examiner-Paul A. Sacher Attomey-Frank R. Trifari [30] Application Priority Data March 7, 1970 Netherlands ..7003279 71 ABSTRACT Y A punctiform cathode for detachable tubes consisting UsS- Cl. l, of a plate. which-is the tops of one or more V-shaped filaments. [51] Int. Cl ..H0lj 17/06, H01 1/05, H01 1/15 'ClainlsADrawing Figures P'A'TENTEDsEPSmIz I 7 3.689.794 I Fig.4 Q

v INVENTOR.

JOHANNES VAN ESDONK I BY I iawa I an heated too strongly.

PUNCTIFORM CATIIODE, IN PARTICULAR SUITABLE FOR DETACHABLE ELECTRIC DISCHARGE TUBES The invention relates to. a punctiform cathode, particularly suitable for use in detachable electric discharge tubes, for example, electron diffraction tubes, electron microscopes and similar tubes, which have to be dismantled several times after use and in which the cathode, after having been in operation, is again contacted with air, or in tubes in which heat-sensitive parts are present and which therefore have to be evacuated without strong heating and can hence contain gas particles, for example, camera tubes, and the like.

In general, punctiform cathodes are used in the said tubes which consist of a hairpin-like or V-shaped bent filament in which the strongly bent part or the top serves as a cathode and is usually arranged close to or in a small aperture of a plate-shaped electrode.

In order to avoid a voltage drop in the part operating as .cathodeand hence to obtain a punctiform equipotential cathode, it is know, for example, from British Pat. Specification No. 455,351, to weld on the top of a hairpin-like filament a plate operating as the cathode and to provide an emissive oxide layer on said plate. v

However, such an oxide cathode is not suitable for the above-mentioned tubes, since the cathode, after the formation, may no longer contact air, since in that case the oxide layer is spoiled beyond repair. The same holds good for tungsten-thorium cathodes.

Therefore, only pure metal cathodes, such as tungsten cathodes, are to be considered for the said tubes. However, it is not possible to secure a plateof tungsten as a cathode on a hairpin-like tungsten wire, since the temperature of the filament must then be inadmissibly high to bring the plate at the emission temperature. The use as a cathode of the bent part of the hairpin-like filament as was done so far, has the additional drawback that due to the curvature of the filament no constant distance is obtained between the actual cathode and a following electrode.

The said drawbacks can be avoided in a cathode for electric discharge tubes, consisting of a metal plate which is secured to the bent part of the V-shaped filament if, according to the invention, the filament consists of tungsten and the plate consists of tantalum.

Since the emission temperature of tantalum is lower than that of tungsten, namely l700 to 1800 C, the

plate can be heated to the emission temperature by a tungsten filament without the filament having to be When a low heating energy is used, the filament must be very thin, however. It has been found that in many cases the temperature gradient along the filament strongly increases from the plate as a result of the comparatively poor heat conductivitythrough the tungsten filament to the plate, as a result of which the temperature of the filament at some distance from the plate can increase too high and the filament at thatregion fuses prematurily. It is found possible to strongly reduce this drawback by using several V-shaped filaments and connecting them with their tops to the tantalum plate. By

. suitable proportioning of the V-shaped filaments, the

heating energy can remain the same. If desirable, the

V-shaped bent heat conductor may be in the form of a ban T e invention will now be described with reference to the accompanying drawing, in which:

FIGS. 1, 2, 3 and 4 show various embodiments of cathodes according to the invention.

In FIG. 1, reference numeral 1 denotes a V-shaped tungsten filament which is secured to current supply pins 2. The pins 2 are sealed in a glass head 3. On the top of the V-shaped filement 1 a tantalum plate 4 is welded or soldered. The plate 4 may be circularor rectangular and have a diameter or diagonal of, for example, 1 mm. The thickness of the plate 4 is, for example, 10p. The diameter of the filament 1 is, for example, 250m.

together withthat of the filament '1, is welded to the plate 4. By suitable proportioning it may be achieved that the two filaments 1 and 5 of FIG. 2 together absorb as much heating energy as the single wire 1 in FIG. 1.

Instead of at one point, the filaments 1 and 5 may also be secured to the plate 4 separately as is shown in FIG. 3. Furthermore, more than two filaments may be .used. a

Instead of two or more filaments, one or more bandshaped filaments 7 may also be used as is shown in FIG.

4. The supporting rods may be sealed in a glass or ceramic disc 6. Instead of separate current supply pins for each filament, two supply pins may be used, for ex-- 4 ample, and the filaments may be secured thereto, for

. example, by means of transverse rods. 

1. A cathode for electric discharge tubes, comprising at least one V-shaped filament of tungsten and a plate consisting of tantalum secured to the bent part of each filament.
 2. A cathode as claimed in claim 1, wherein the top of each filament is secured to the tantalum plate.
 3. A cathode as claimed in claim 1 wherein at least one filament is in the form of a band. 